I agree totally with the last post about getting the pancreatic elastase stool test. Deficiency of the pancreatic enzymes cause malabsorption, greasy stool, diarrhea, weight loss and prevents you from digesting fats, so fat soluble vitamins are deficient too. These can be measured in serum. A gastroenterologist would order this test or your pcp if familiar with this condition which is not terribly common. It’s treated by taking pancreatic enzymes with meals. I have this condition so recognize your symptoms.